Quantitative Index of the Relative Ease of Formation and σ-Bonding Strength of N-Heterocyclic Carbenes.

An energy-based index of the ease of N-heterocyclic carbene (NHC) formation either by deprotonation of precursor salts to give neutral NHCs or deprotonation of heterocyclic mesomeric betaines to give anionic NHCs is described. This index (CREF; Carbene Relative Energy of Formation), which is easily calculated using DFT methods, also gives a quantitative measure of the relative σ-donor strength of NHCs. CREF index values for a wide range of known and unknown NHC ring systems are reported and their significance discussed.